Sorption of trivalent europium Eu(III) by Na+-substituted bentonite: Mechanistic insight and stabilization effect

The sorption of Eu(III) by Na⁺-substituted bentonite (Na-bentonite) was investigated as a function of pH and NaNO₃ concentration ([NaNO₃]₀). At pH < ∼7.5, Eu(III) sorption decreased with the increasing [NaNO₃]₀, whereas at pH > ∼7.5, it remained nearly complete, independent of [NaNO₃]₀. Our thermodynamic model indicated that the sorption at pH < ∼7.5 occurred via a combination of cation exchange and surface complexation, with the former diminishing as [NaNO₃]₀ increased. Meanwhile, the sorption at pH > ∼7.5 was primarily due to surface complexation. By X-ray diffraction, the incorporation of hydrated Eu(III) in the interlayers of montmorillonite increased its lattice spacing and crystallinity along the c-axis, where cation exchange was predominant. Also, Eu L_III-edge X-ray absorption spectroscopy revealed that the sample dominated by cation exchange had an absorption edge energy and coordination structure similar to aqueous Eu(III), indicating outersphere complex formation. In other cases, Eu(III) sorption was characterized by innersphere complexation, as indicated by increased covalency and the presence of more pronounced second coordination shells. Importantly, the analysis of dissolved Si and Al suggested that the increased stability of Na-bentonite was likely due to the surface complexation of Eu(III) with aluminol groups at the edges, especially at higher surface coverages. Given its high sorption capacity for Eu(III) and stabilization effect mediated by sorption, Na-bentonite could be serve as an effective backfill material and migration barrier for containing actinides in nuclear waste repositories.